the cost of a used 96 a6quattro?

hi everyone,
i'm new to the list and this is my first question, so be gentle with me :-)
the only cars ive ever had have been used audis. first a 1980 5000s, then a
1982 5000s which ive been driving the past 10 years. now i'm finally ready to
purchase my dream audi, but i'm really confused about price.
I've seen the 1996 A6 Quattro that i want to buy, but the problem is trying to
figure out the real market value of the car. i've looked at edmunds, kbb,
carprices.com, and every internet site for car pricing i can find. but the
price range on the cars is variant by like 2000-4000 depending on the site and
their calculations. the guy is asking $24, 800 for the car (which seems high
compared to all the top prices i've seen on any sites) - it's fully loaded,
has the bose stereo, leather, every option that i can tell, and has 43,000K on
it. I'd appreciate any help i can get from the list subscribers in trying to
figure out a decent and reasonable offer to give the seller. thanks so much
for any help - lydia